1. Field of the Invention
This invention relates to a liquid crystal device applicable to various displays, and more particularly to a liquid crystal device having an alignment-controlling film formed by applying an aqueous solution containing polyvinyl alcohol (which will be hereinafter referred to as "PVA") and titanium alkoxide, followed by heating.
2. Description of the Prior Art
PVA has been so far known as a material capable of forming a film capable of acting as an alignment-controlling film by heating at a relatively low temperature without using any toxic and inflammable organic solvent. PVA is soluble in water and thus safety control of working can be made easier by using water as a solvent. Furthermore, PVA has many advantages because the film can be formed by heating at a low temperature. For example, liquid crystal display devices using a plastic substrate or transistors can be prepared without any difficulty in spite of their low heat resistances. However, when an aqueous PVA solution is applied as such to a substrate, followed by heating, the resulting alignment-controlling film is hygroscopic and thus a liquid crystal display device of high durability cannot be obtained. Thus, researches have been so far made to remedy such a drawback of the film and provide a liquid crystal display device utilizing the advantages of PVA.
One of the so far proposed improvements is to form an alignment-controlling film by applying an aqueous PVA solution containing 0.5 to 2 parts by weight of titanium alkoxide per part by weight of PVA, followed by heating (Japanese Patent Laid-open Publication No. 45426/1984), where the hygroscopicity of the film is controlled by cross-linking reaction between PVA and titanium alkoxide by adding titanium alkoxide to PVA in such a range as not to impair to alignment control or insulation of the PVA film. The hygroscopicity of the PVA alignment-controlling film can be considerably improved by the addition of titanium alkoxide to the aqueous PVA solution but the hygroscopicity still remains to some extent. Thus, the alignment-controlling film must be provided only at the inner part of the substrate surface surrounded by a sealant in the sense of shutting off the external moisture sufficiently, thereby helping to increase the moisture resistance. To this end, at first the entire surface of a substrate is coated with an aqueous solution of PVA and titanium alkoxide by spinning, dip-coating, etc. to form a film, and then the film portion formed at the peripheral part of the surface is removed by etching. However, chromic anhydride to be used in the etching is hard to handle and there arise other problems such as a risk of incomplete etching etc.
On the other hand, application of an aqueous solution of PVA and titanium alkoxide to the desired area on a substrate by printing has been proposed. However, the aqueous solution of PVA and titanium alkoxide has a poor printability, and no uniform film can be obtained by printing.
As a result of evaluation of liquid crystal display devices having the alignment-controlling film formed from the said aqueous solution by the present inventors, it has been found through moisture resistance tests that in liquid crystal devices having electrodes arranged in a fine clearance there appears such a phenomenon that the adjacent interelectrode spacing acts as a pseudo-electrode to light a non-display segment.
That is, when an interwiring spacing in a display segment is made to be about 100 .mu.m to about 50 .mu.m to obtain a small liquid crystal device of high density, and when the insulation of the substrate surface at the interelectrode clearance is lowered even slightly due to the moisture absorption of the alignment-controlling film, the substrate surface at the spacing turns to a pseudo-electrode, giving a potential difference between this pseudo-electrode and the common electrode above the spacing. As a result, the wiring at the non-display segment is brought into a lighted state. Such a state is called "deterioration due to black stripe" (wiring being so narrow to cause stripe-like deterioration). Against the properly lighted display segment, the unlighted substrate surface around the segment acts as a pseudo-electrode and a lighted state is brought about between this pseudo-electrode and the common electrode provided in a large size in advance than that of the segment. This is called "deterioration due to blurring" (lighting being extended over the proper lighted display segment).
These black stripes and blurring appear at a very restricted distance, and thus are usually no problem at all in the ordinary display of a relative large image element, and have been so far unnoticed. As a result of researches to obtain fine patterns as a display of high reliability, the present inventors have moticed these phenomena for the first time.
In case of a liquid crystal device for color display, a color filter is usually provided on an electrode plate, where a color filter made from a gelatin film or PVA film dyed with a dye is used.
In case of using the gelatin film as a dyeing medium film, a liquid crystal display device with an overall satisfaction for the durability, display characteristics, etc. with relation to an alignment-controlling film serving also as a protective film to be laid on the gelatin film is hard to obtain. That is, the gelatin film as a dyeing medium film cannot be used together with an alignment-controlling film serving also as a protective film, which is formed by using an organic solvent or by curing at a high temperature, because of a low solvent resistance and a low heat resistance of the former. Thus, an alignment-controlling film with good adhesion is hard to obtain and also deterioration of adhesion is liable to occur between these two films.
On the other hand, in case of using a PVA film, it has been proposed to use a film with an improved moisture resistance by adding titanium alkoxide to PVA as an alignment-controlling film serving also as a protective film. In that case, the adhesion can be improved between the dyeing medium film and the alignment-controlling film serving also as a protective film, because they are made from the same material, but there are still the following problems. That is, though the moisture resistance can be improved by adding titanium alkoxide to PVA, the hygroscopicity is not completely eliminated thereby. When a PVA film is tightly laid on a modified PVA film with an improved moisture resistance by adding titanium alkoxide to PVA, the moisture absorption of the PVA film containing titanium alkoxide is accelerated, to the contrary, due to the hygroscopicity of the PVA film. As a result, the insulation of an alignment-controlling film serving also as a protective film is lowered, and satisfactory durability and display characteristics cannot be obtained. These problems have been overlooked, because the PVA film containing titanium alkoxide has been only used as an alignment-controlling film. It is the present inventors that have solved these problems for the first time as a result of the researches for using it also as a protective film.